diff --git a/ckeditor.api.php b/ckeditor.api.php
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+<?php
+
+/**
+ * @file
+ * Documentation for CKEditor module APIs.
+ */
+
+/**
+ * @addtogroup hooks
+ * @{
+ */
+
+/**
+ * Provides a list of CKEditor plugins.
+ *
+ * Each plugin for CKEditor must provide an array of properties containing
+ * information about the plugin. At minimum, plugins must provide a path and
+ * file location so that CKEditor may add the plugin. Available properties for
+ * each plugin include:
+ *
+ * - location: Required for all external plugins. String path to the plugin
+ *   directory relative to the Drupal installation root. Do not include a
+ *   trailing slash.
+ * - file: Required for all external plugins. String file name of the plugin in
+ *   the "location" directory.
+ * - internal: Boolean value indicating if the plugin is part of the compressed
+ *   CKEditor library package and already loaded on all instances. If TRUE,
+ *   the "location" and "file" properties are not needed.
+ * - css: An array of CSS files that should be added by CKEditor. These files
+ *   are used only when CKEditor is using an iframe wrapper around its content.
+ *   If a plugin needs to include CSS for inline and iframe versions, it should
+ *   add its CSS via CKEditor's JavaScript CKEDITOR.addCss() method.
+ * - enabled callback: String containing a function name that can determine if
+ *   this plugin should be enabled based on the current editor configuration.
+ *   See the hook_ckeditor_PLUGIN_plugin_check() function for an example.
+ * - buttons: An array of buttons that are provided by this plugin. Each button
+ *   should by keyed by its CKEditor button name, and should contain an array
+ *   of button properties, including:
+ *   - label: A human-readable, translated button name.
+ *   - image: An image for the button to be used in the toolbar.
+ *   - image_rtl: If the image needs to have a right-to-left version, specify
+ *     an alternative file that will be used in RTL editors.
+ *   - image_alternative: If this button does not render as an image, specify
+ *     an HTML string representing the contents of this button. This alternative
+ *     will only be used in the administrative section for assembling the
+ *     toolbar.
+ *   - attributes: An array of HTML attributes which should be added to this
+ *     button when rendering the button in the administrative section for
+ *     assembling the toolbar.
+ *   - multiple: Boolean value indicating if this button may be added multiple
+ *     times to the toolbar. This typically is only applicable for dividers and
+ *     group indicators.
+ *   - required_tags: If this button requires certain HTML tags to be allowed,
+ *     specify an array of tags.
+ * @return array
+ *   An array of plugin definitions, keyed by the plugin name.
+ *
+ * @see ckeditor_ckeditor_plugins()
+ * @see hook_ckeditor_PLUGIN_plugin_check()
+ * @see \Drupal\editor\Plugin\EditorBase
+ */
+function hook_ckeditor_plugins() {
+  // The drupalcaption plugin provides consistent behaviors for image captions.
+  $plugins['myplugin'] = array(
+    'path' => drupal_get_path('module', 'mymodule') . '/js/myplugin',
+    'file' => 'plugin.js',
+    'css' => array(drupal_get_path('module', 'mymodule') . '/css/myplugin.css'),
+    'enabled callback' => 'mymodule_myplugin_plugin_check',
+  );
+
+  return $plugins;
+}
+
+/**
+ * Modify the list of available CKEditor plugins.
+ *
+ * This hook may be used to modify plugin properties after they have been
+ * specified by other modules.
+ *
+ * @param $plugins
+ *   An array of all the existing plugin definitions, passed by reference.
+ *
+ * @see hook_ckeditor_plugins()
+ */
+function hook_ckeditor_plugins_alter(array &$plugins) {
+  $plugins['someplugin']['enabled callback'] = 'mymodule_someplugin_enabled_callback';
+}
+
+/**
+ * Modify the list of CSS files that will be added to a CKEditor instance.
+ *
+ * Modules may use this hook to provide their own custom CSS file without
+ * providing a CKEditor plugin. This list of CSS files is only used in the
+ * iframe versions of CKEditor.
+ *
+ * Note that because this hook is only called for modules and the active theme,
+ * front-end themes will not be able to use this hook to add their own CSS files
+ * if a different admin theme is active. Instead, front-end themes and base
+ * themes may specify CSS files to be used in iframe instances of CKEditor
+ * through an entry in their .info file:
+ *
+ * @code
+ * ckeditor_stylesheets[] = css/ckeditor-iframe.css
+ * @endcode
+ *
+ * @param $css
+ *   An array of CSS files, passed by reference. This is a flat list of file
+ *   paths relative to the Drupal root.
+ * @param $editor
+ *   The editor object as returned by editor_load(), for which these files are
+ *   being loaded.
+ * @param $format
+ *   The corresponding text format object as returned by filter_format_load()
+ *   for which the current text editor is being displayed.
+ *
+ * @see _ckeditor_theme_css()
+ */
+function hook_ckeditor_css_alter(array &$css, $editor, $format) {
+  $css[] = drupal_get_path('module', 'mymodule') . '/css/mymodule-ckeditor.css';
+}
+
+/**
+ * @} End of "addtogroup hooks".
+ */
+
+/**
+ * Enabled callback for hook_ckeditor_plugins().
+ *
+ * Note: This is not really a hook. The function name is manually specified via
+ * 'enabled callback' in hook_ckeditor_plugins(), with this recommended callback
+ * name pattern. It is called from
+ * \Drupal\ckeditor\Plugin\editor\editor\CKEditor:getJSSettings().
+ *
+ * This callback should determine if a plugin should be enabled for a CKEditor
+ * instance. Plugins may be enabled based off an explicit setting, or enable
+ * themselves based on the configuration of another setting, such as enabling
+ * based on a particular button being present in the toolbar.
+ *
+ * @param object $editor
+ *   An editor instance as returned by editor_load(). The editor's settings may
+ *   be found in $editor->settings.
+ * @param string $plugin_name
+ *   String name of the plugin that is being checked.
+ *
+ * @return boolean
+ *   Boolean TRUE if the plugin should be enabled, FALSE otherwise.
+ *
+ * @see hook_ckeditor_plugins()
+ * @see \Drupal\ckeditor\Plugin\editor\editor\CKEditor:getJSSettings()
+ */
+function hook_ckeditor_PLUGIN_plugin_check($editor, $plugin_name) {
+  // Automatically enable this plugin if the Underline button is enabled.
+  foreach ($editor->settings['toolbar']['buttons'] as $row) {
+    if (in_array('Underline', $row)) {
+      return TRUE;
+    }
+  }
+}